This invention relates to a wavelength-division-multiplexing optical transmission system and control method thereof, and particularly to a wavelength-division-multiplexing optical transmission system and control method thereof that comprises an optical-node apparatus that controls the optical level based on the number of multiplexed wavelengths,
Wavelength-division-multiplexing (WDM) optical transmission that multiplexes a plurality of optical signals having different wavelengths in one optical fiber is a very effective method for performing a large volume of optical fiber communication. In the mainline of the wavelength-division-multiplexing optical transmission system, constant optical gain control is performed for the optical amp for prioritizing the level evenness characteristic of a wavelength. By performing this constant-optical-gain control it is possible to control the power per one wavelength (one channel) to be a fixed value even when the number of multiplexed wavelengths (number of channels) changes.
(A) of FIG. 14 shows the construction of part of the mainline of a wavelength-division-multiplexing optical transmission system that uses an optical amp for performing this kind of constant-optical-gain control. In the mainline of the wavelength-division-multiplexing optical transmission system, the transmission path is connected in a ring shape, and optical add/drop multiplexers (OADM) 1a, 1b are located at suitable locations. The OADM multiplexes data that is input from a user terminal (not shown in the figure) onto the optical signal of the mainline, and transmits it, or splits the multiplexed optical signal of the mainline and extracts data for a user and transmits it to the user terminal. A post amp 2a, 2b that performs constant-optical-gain control is connected to the output side of each OADM 1a, 1b, a pre amp 3a, 3b that performs constant-optical-gain control is connected to the input side of each OADM 1a, 1b, and an optical transmission path 4 is connected between the post amp 2a and pre amp 3b. By using an optical amp for performing constant-optical-gain control in this way, it is possible to even out the wavelength characteristic, however, as shown in (B) of FIG. 14, it is not possible to handle the fluctuation in the pre-amp output level (OADM input level) caused by the fluctuation of transmission-path loss La, Lb, Lc.
Therefore, while maintaining a wavelength-evenness function, it further becomes necessary to have a level-control function for quickly making the output level of the amp constant when level fluctuation occurs. Therefore, an optical-transmission system has been proposed as a first prior art that comprises an optical amplifier having a constant-optical-gain-control function and level-control function, in which the optical amplifier normally performs constant-gain control, and at a constant period performs level control (refer to JP11-121848A). Also, as shown in (A) of FIG. 15, an optical-transmission system has been proposed that has a constant-optical-gain-control function and constant-level-control function in the pre amp 3b. In (A) of FIG. 15, the same reference numbers are given to parts that are identical to those in (A) of FIG. 14.
As shown in (B) of FIG. 15, according to the optical amplifiers of the prior art, even when there is loss fluctuation La, Lb, Lc in the transmission path, it is possible to maintain the output of the pre amp 3b, or in other words, it is possible to keep the input level to the OADM constant.
In wavelength-division-multiplexing optical transmission, when the number of wavelengths (number of channels) that are multiplexed changes, the optical power of the multiplexed wavelengths changes, and thus the optical-input level to the optical amplifier changes. However, in the prior constant-level control shown in FIG. 15, it is not determined whether the fluctuation in the optical-input level is due to a change in the number of channels, or due to loss fluctuation of the transmission path. Therefore, in the prior constant-level control, control is performed to bring the output level to a set level even when the number of multiplexed wavelengths (number of channels) decreases, and thus the input level becomes low, so the output level per wave becomes too large, and signal-transmission error occurs. Also, in the prior constant-level control, control is performed to keep the output level constant even when the number of channels increases, so the output level per wave becomes low and signal-transmission error occurs.
Therefore, a control method for a wavelength-division-multiplexing optical transmission system has been proposed as a second prior art in which level control is performed according to the number of channels, and constant-gain control is performed temporarily only when there is a notification that the number of channels has changed, and after that constant-level control is performed according to the number of channels after the change (see JP2000-20111A).
According to this second prior art, a certain amount of time T is required from the time that the number of channels actually changes until constant-gain control starts. During this time T, the optical amplifier of this second prior art controls the output level so that it is at a level that corresponds to the number of channels before a change in channels. In this case, when the time T is long, or when the level-recovery speed of the optical amplifier is fast, the output level during that time T becomes a value that is quite far from the proper level that corresponds to the actual number of channels. For example, when the number of channels decreases, the output level becomes a value that is quite larger than the proper level, and when the number of channels increases, the output level becomes a value that is quite lower than the proper level. Therefore, during the period until level control starts according to the actual number of channels, and particularly, during the period that constant-gain control is performed, signal-transmission error occurs.